Continuous time interval measurements on a signal provide a way to analyze characteristics of the signal in the modulation domain, i.e., the behavior of the frequency or phase of the signal versus time. This is different than classic ways of measuring and displaying data about signals. An oscilloscope shows amplitude versus time: the time domain. A spectrum analyzer shows amplitude versus frequency: the frequency domain.
Continuous time interval measurements make it simpler to study dynamic frequency behavior of a signal: frequency drift over time of an oscillator, the frequency hopping performance of an agile transmitter, chirp linearity, and phase switching in radar systems.
A histogram is a picture of the distribution of measurement results as a function of a selected variable. For time interval results, for example, a histogram might display the distribution of a set of measurements over a range of time durations.
Generally, compiling data into a histogram using software processing is very slow because there is so much processing time between blocks of measurements. For example, in the Hewlett-Packard 5371A Time Interval Analyzer data is acquired, processed, and added to a growing histogram one block at a time in blocks of up to 1000 measurements. There is about 6 seconds for every 1000 measurement block. At this rate, it would take about 69 days to accumulate 1 billion measurements.
Compiling the histogram using hardware greatly improves the rate at which data can be histogrammed. With the histogram circuit of the invention, data can be collected at a rate of at least 10 MHz. At this rate, it would take only 100 seconds to acquire 1 billion measurements. This is 60,000 times faster than the histogram rate of the software method.
When histogramming, the stored bin data must be incremented each time a new measurement occurs. For non-incrementing RAMs, this requires that the stored data be read, incremented, and then written back into the RAM. Using a standard single port RAM, this requires that two memory cycles plus an increment calculation be performed in the time between data acquisitions. This severely limits the speed at which histogramming can be performed.
Event triggering is used to capture the data for a particular event of interest. A circuit is programmed to produce a trigger signal on the occurrence of some aspect of the input waveform that is characteristic of the event to be captured. In response to the trigger signal, a memory controller stops the flow of data into memory, so the memory holds the last iteration of data written into memory, which is the data from the event of interest. Depending on the delay from the trigger signal to the stop writing command, data prior to, subsequent to, or surrounding the trigger event will be captured.
There are a variety of conventional triggering modes, based on the amplitude or the slope of the input waveform. For example, by using maximum and minimum limit values (hystersis bands), the trigger can be set to occur when the input signal crosses a threshold voltage in a positive direction, a negative direction, or either direction.